In such an indicator light, each facet of the mirror has a form such that it is adapted to direct the reflected light horizontally and vertically, so that the facet by itself covers at least part of the field of illumination of the indicating light. As a result, when the indicating light is observed within this field of illumination, and whether or not the cover lens is smooth or, as may be the case, includes reliefs such as decorative elements, which have little influence on the behaviour of the radiated light, it is found that there occur a number of secondary light sources which are spaced apart at regular intervals. These secondary light sources are created by the various facets of the mirror, and they give the indicating light an aspect which is sometimes called a "diamond aspect". This diamond aspect is very attractive to styling designers.
An indicating light with these features is quite easy to design when each of the facets has, in orthogonal projection on the optical axis, horizontal and vertical sides, because each facet can then easily be designed in such a way as to distribute the light projected in a rectangle which is selected so as to be equal or similar to the required field of illumination, the latter being also generally of rectangular form.
However, this constraint regarding the edges of the facets involves some penalties for the stylist. In this connection, the stylist may want the contours of the facets to be, not horizontal and vertical, but with oblique orientations instead, following for example the contour lines of the neighbouring parts of the bodywork of the vehicle. But if the facets are to have such contours, it becomes impossible to make them able to cover a projected field of illumination corresponding closely enough to the required rectangular field of illumination. In that case, when the observer, outside the vehicle, is close to the edges of the field of illumination, or moves progressively away from this field, some of the secondary light sources cease to be visible to the observer, while others remain visible. From the styling point of view this effect is regarded as being extremely undesirable.